My memory isn't what it used to be, so I dug up my Livejournal post from 2005 in which I recalled 09/11/2001, to share my experience that was now a full 20 years ago. The fact that this event is basically ancient history to even some young adults truly boggles my mind, as it seems to me almost like it happened yesterday.
(Additions in  brackets are notes I've added today to better explain.)
It was my senior year, and I was driving to school. It was roughly 7:50 in the morning (CST). I was listening to a morning show on the radio and they suddenly switched topics to tell about a plane crashing into the World Trade Center. Partly, I believe, due to the disbelieving tones of the morning DJs, I thought it was simply another joke.
An hour or so later, during band practice, the principal came over the loudspeakers to announce what was happening, but we were playing and couldn't make out what he'd said, but his tone was somber, and there was a vague impression that he'd said to turn on the classroom televisions. That's when I realized what I'd heard on the radio had been true, and I had the grim task of explaining to the 100+ people in the room what I knew of what had happened [though I didn't know any details yet, just that a plane had hit a building in New York].
At approximately 8:55am (CST), we were released from band. As custodians, lunch ladies and students watched in horror [on the commons area televisions], the South Tower collapsed before our very eyes (8:59am CST).
In my second period webmastering class, lessons were forgotten as everyone scrambled to the news sites for whatever information we could find. This was mostly in vain; the local ISPs and the news sites themselves were so clogged from unprecedented web traffic [the internet was still quite young at this time] that they were rendered virtually useless.
Approximately a half hour into second period, at 10:28, we watched the North Tower collapse.
Lesson plans were almost universally put aside and tardies mostly ignored as we spent the remainder of the day watching the most horrific event in US history any of us teenagers had ever witnessed.
I remember going home after school to find my mom watching the news.
I remember trying and largely failing to process the concept that we'd been attacked on American soil and so many people were just gone.
I remember stopping for gas that evening between class and evening band practice, the prices already starting to go up. They'd never go back down to the prices we'd had before.
I remember wondering at the fact that band practice was going on as scheduled despite what had happened. At least one kid, even in our little town, knew someone who'd gone missing in the attacks. I never did find out what happened to their loved one.
I remember seeing things in the coming days, months, years, that would never leave me; photos and eventually even video of planes hitting buildings, of desperate people leaping from the buildings, of mountains of wreckage, of people fleeing the clouds of smoke and debris from the collapsing buildings.
I remember, and as long as I live, I'll never forget.